Try twisting the boot first to loosen it up. If you get the boot off use dielectric greese to help prevent it from seizing to the plug for the next time. I struggled with my E320 the first time, it took me 1.5 hours to replace the plugs.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles